Friction and wear processes in piston rings

It has been recognised that a large part of the top piston ring wear of an ic engine takes place in boundary lubrication around top dead centre (tdc) position. A quantitative assessment of the friction behaviour using actual piston ring and cylinder liner under conditions close to tdc has been made. The factors responsible for wear under these conditions have been identified as surface temperature, peak combustion pressure, total energy on the wearing surfaces and other physical properties of the material under sliding     

On the hydrodynamic lubrication analysis of piston rings

A theoretical hydrodynamic lubrication analysis of piston rings is presented. A numerical scheme is developed to solve the Reynolds' equation and the load equilibrium equation simultaneously. The hydrodynamic effects are examined in detail by considering solutions to the problem of lubricating the top compression ring in a two-stroke diesel engine. The minimum film thickness is computed throughout a complete cycle. The pressure profiles and film shapes at different crankshaft angles are presented. In the method proposed, the camtation boundary condition within the lubricated conjunction is included in the analysis.     

Effects of lubricant viscosity on the mixed lubrication of a piston ring pack in an internal combustion engine

A theoretical analysis is presented of the mixed lubrication of a piston ring pack. The analysis comprises Patir and Cheng's average flow model and Greenwood and Tripp's asperity interaction model, and is developed to consider the shear thinning effect of a non‐Newtonian fluid, multigrade lubricant. The friction characteristics of the piston ring pack for both monograde and multigrade lubricants are investigated. It is found that a decrease in the lubricant viscosity is effective in reducing the friction loss, although this increases the boundary friction at the beginning of the expansion stroke. The friction characteristics are markedly affected by the shear thinning effect when multigrade lubricants are used.     

发动机缸套-活塞环摩擦磨损特性试验研究

利用缸套-活塞环摩擦磨损试验台研究了速度，温度，载荷，供油等因素对缸套-活塞环系统摩擦磨损特性的影响。试验结果表明，缸套-活塞环摩擦副在发动机工作循环中润滑状态不断发生变化。在试验条件下，温度对摩擦磨损有显著影响，载荷和速度对摩擦力的影响较小。     

织构化活塞环/缸套的瞬态热流体动力润滑分析

考虑温升对活塞环/缸套流体动力润滑的影响,联立广义Reynolds方程、膜厚方程、载荷方程、能量方程及热传导方程,建立织构化活塞环/缸套的瞬态热流体动力润滑模型;采用多重网格法和逐列扫描法进行求解,探究织构参数对热流体润滑的影响。结果表明:摩擦引起的温度升高使得最小油膜厚度和最大油膜压力变化趋势及大小均发生显著的变化,这表明在对该摩擦副进行动力润滑分析时必须考虑温升的影响;织构面密度和深度对最小油膜厚度、最大油膜压力及最大油膜温度随曲柄转角的变化趋势没有明显的影响,但对它们的值的大小产生不可忽略的影响,其中在做功冲程,小织构面密度和织构深度对应较小的最小油膜厚度和最大油膜温度,较大的最小油膜厚度。研究表明:在一定范围内,小的织构面密度和织构深度具有更优的燃油经济性,反之则具有更优的润滑可靠性。     

Initial sliding wear on piston rings in a radial piston hydraulic motor

The initial sliding wear of boundary lubricated piston rings used in a hydraulic motor is studied in terms of the changes in mass, form and surface roughness. The piston ring in a hydraulic motor makes an important contribution to high volumetric efficiency by properly sealing the cylinder bore and piston. The results show that the wear on the piston ring in this particular test rig takes place at the top of the asymmetric crowning at the outer surface contacting the cylinder bore. Initially, the roughness amplitude decreased rapidly, and had decreased by one-third after sliding 10 m. The dominant wear mechanism was mild wear. Abrasive wear also clearly influences the amount of wear.     

Piston ring friction loss behaviour for motored and fired reciprocating engines

A previously developed piston ring lubrication model has been further extended so that the piston ring frictional losses can be predicted in both hydrodynamic lubrication and metal-to-metal contact regions for various engine operating conditions. Ring friction results for two engine types are presented for both hot motoring and engine firing conditions. The hot motoring predictions were found to be in good agreement with tests. Results show that when the engine is motored, piston ring friction losses in the hydrodynamic lubrication region predominate. If the engine is fired, the losses in the metal-to-metal contact region become dominant due to high gas pressure and temperature effects. Ring friction loss can be significantly reduced by using low tension rings with a correct ring sliding face profile.     

Tribological performance of three advanced piston rings in the presence of MoDTC-modified GF-3 oils

The piston assembly (piston ring and cylinder bore) is one of the key parts of the internal combustion (IC) engine. Its performance will directly determine the performance of the whole engine. The piston assemblys tribological performance will be influenced by both its mechanical properties and the tribochemical interactions that take place on their surfaces. In this paper, three kinds of advanced stainless steel piston rings with a single nitrided layer, CrN coating on the nitrided layer and a B₄C and CrN binary-layer coating on the nitrided layer, respectively, were employed. Their frictional behavior and wear performance, when sliding against the cast iron cylinder bore materials lubricated with two kinds of GF-3 category mineral-based engine oils (one of them blended with MoDTC friction modifier), were investigated on a SRV tribotester. The test conditions were set and maintained to simulate engine-operating conditions. SEM (Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray spectroscopy) were employed to characterize the morphology and elemental composition of the wear tracks. Tribotests and analysis results indicate that changes in both the mechanical properties of the tribomate (piston coatings) and tribochemical interactions (formulation of engine oils) have an impact on the tribological performance of the piston assembly. Tribochemical interactions will have a more obvious influence on friction coefficients while the mechanical properties of the tribomate have a more obvious influence on wear.     

考虑结构动态变形的缸套-活塞环润滑建模分析

缸套在燃烧冲击和活塞敲击激励下会产生接近表面粗糙度的动态变形,极有可能影响缸套-活塞环组件间的摩擦润滑过程。为了研究缸套动态变形潜在的影响,将动力学仿真获取的缸套内表面的动态变形经过处理后导入到润滑模型中,同时采用数值积分计算的方式对油液压力应力因子和剪切应力因子进行实时计算求解,使仿真计算更加符合实际情况。通过搭建同时考虑缸套变形与油液剪切特性影响的改进润滑模型,计算得到整个工作循环内活塞环上的最小油膜厚度和摩擦力曲线。结果表明:考虑缸套动态变形后的最小油膜厚度和摩擦力曲线出现了明显的波动,而且考虑缸套动态变形后的摩擦力比未考虑之前出现了明显下降。     

Tribological issues in reducing piston ring friction losses

In this paper improvement in fuel economy by reducing piston ring friction is studied and reduction of piston ring tension and the use of two-ring packages are found to be effective. However, these can be accompanied by adverse effects such as excessive wear and increasing oil consumption, and measures to overcome these problems are studied in detail.     

Studies on the tribological behaviour of two oil-soluble molybdenum compounds under reciprocating sliding conditions

The use of oil-soluble organo-molybdenum compounds in engine oils can help in fuel conservation by reducing engine friction and increase durability by improving wear characteristics. The tribological behaviour of molybdenum dialkyldithiophosphate (MoDDP) was studied with a SRV Optimal tester under reciprocating sliding conditions. The studies were conducted under nonconformal contact conditions on En-31 steel and conformal contact conditions on piston ring and liner materials. The results indicated that the frictional behaviour of these additives is shear sensitive. Shear sensitivity is influenced by materials, operating conditions and the interaction of MoDDP with additives present in the oil. MoDDP when used in base oil was found to increase the scuff resistance of ring and liner materials. The use of the additive reduced friction and wear (running in as well as normal) under nonconformal contact conditions, while under conformal contact conditions its effect was selective and limited.     

On the effective length of the top piston ring involved in hydrodynamic lubrication

This paper describes a simple analytical model for top piston ring lubrication. The model is constructed using the open‐end boundary condition hypothesis, with an assumption of minimum hydrodynamic pressure in the effective ring length equal to the saturation pressure. The analysis confirms that not all of the ring length is involved in the case of hydrodynamic lubrication, and the effective length is found to be less than 40–50% under some operating conditions. The results are in agreement with previous investigations.     

内燃机缸套-活塞环润滑理论模型概述

从基本概念、基础理论、发展阶段、研究内容、应用领域及发展趋势等方面综述了缸套-活塞环润滑理论模型研究概况。     

极限环境温度对柴油机气缸套磨损的影响

为了研究不同环境温度对柴油机寿命的影响,以某型柴油机为对象,通过模拟极限环境温度下柴油机实际工况,利用受力分析和热平衡计算得到边界条件,基于雷诺方程和改进的Holm-Achard黏着磨损公式建立润滑磨损数值计算模型.经验证上止点附近模型的计算值与实测值误差不超过5%.计算表明,油膜厚度随外界温度升高而逐渐变薄.高温319K时气缸套径向最大磨损深度为41.21μm,低温230 K时为35.8μm.     

A rig test to measure friction and wear of heavy duty diesel engine piston rings and cylinder liners using realistic lubricants

Laboratory tests to evaluate piston ring and cylinder liner materials for their friction and wear behavior in realistic engine oils are described to support the development of new standard test methods. A ring segment was tested against a flat specimen of gray cast iron typical of cylinder liners. A wide range of lubricants including Jet A aviation fuel, mineral oil, and a new and engine-aged, fully formulated 15W40 heavy duty oil were used to evaluate the sensitivity of the tests to lubricant condition. Test temperatures ranged from 25 to 100 °C. A stepped load procedure was used to evaluate friction behavior using a run-in ring segment. At 100 °C, all lubricants showed boundary lubrication behavior, however, differences among the lubricants could be detected. Wear tests were carried out at 240 N for 6 h at 100 °C with new ring segments. The extent of wear was measured by weight loss, wear volume and wear depth using a geometric model that takes into account compound curvatures before and after testing. Wear volume by weight loss compared well with profilometry. Laboratory test results are compared to engine wear rates.     

Measurement of lubricant flow in a gasoline engine

A residence time of approximately 60 s for lubricant in the piston ring pack of a gasoline engine, running at 1500 rpm, 50% throttle, has been measured by adding a hydrocarbon marker to the oil in the sump and monitoring its build up in oil extracted from the top ring groove. Comparison of the rate of increase of oxidised products in the sump with the level of oxidation in the ring pack allows a rate of flow of oil entering the ring pack from the sump to be calculated as approximately 0.4 cm³ min⁻¹ cylinder⁻¹, with at least 88% returning to the sump and the remainder lost into the combustion chamber. The volume of oil in the ring pack was determined as approximately 0.4 cm³ cylinder⁻¹, approximately one quarter of the free volume available between the top of the oil control ring and the top piston ring.     

外台阶锥形环混合润滑的积分算法及其与差分法的比较

建立考虑到活塞环尺寸、弹力、润滑油黏度、粗糙度等影响因素的外台阶锥形环-缸套系统的混合润滑仿真模型,分别基于积分法和有限差分法求解一维平均雷诺方程,获得整个工作循环内的油膜厚度、油膜压力和摩擦力等参数随转角的变化关系。结果表明,积分法在求解平均雷诺方程时速度较快,程序框架简单明了,但积分法的推导依赖于活塞环型线,而差分算法对活塞环型线依赖性较小;2种算法得到的锥形环上行时的油膜厚度相差较大,这是由于所采用的油膜破裂边界不同,其中差分法采用的Reynolds边界条件相对于积分法采用的半Sommerfeld边界条件在边界的确定上更加准确。     

Influence of Surface Groove Width on Tribological Performance for Cylinder Liner–Piston Ring Components

Surface groove width is of great significance to the performance of the cylinder liner–piston ring (CLPR) with the different surface textures in marine diesel. However, little is understood about the specific application (e.g., the geometric parameters of surface textures and operating conditions) of surface texture in actual marine diesel engines. To address this issue, different surface groove textures including grooves structures with 1-, 2-, 3-, and 4-mm widths were designed based on previous results related to marine diesel engine applications. A series of experimental tests was conducted in a reciprocation tester, and data on the friction characteristics were obtained under different operating conditions. Comparative studies on the friction coefficients, worn surface features, and oil film characteristics were performed. Results showed that the 2-mm groove structure of the cylinder liner was more favorable for improving the wear performances at low speed, whereas a 3-mm groove structure of the cylinder liner was more suitable for improving the wear performance at higher speeds, though its wear performance needs to be improved under high load. These results help to understand the specific application of surface texture on the wear performance of the CLPR pair.     

Tribological evaluation of piston skirt/cylinder liner contact interfaces under boundary lubrication conditions

The friction and wear between the piston and cylinder liner significantly affects the performance of internal combustion engines. In this paper, segments from a commercial piston/cylinder system were tribologically tested using reciprocating motion. The tribological contact consisted of aluminium alloy piston segments, either uncoated, coated with a graphite/resin coating, or an amorphous hydrogenated carbon (a‐C : H) coating, in contact with gray cast iron liner segments. Tests were conducted in commercial synthetic motor oils and base stocks at temperatures up to 120°C with a 2 cm stroke length at reciprocating speeds up to 0.15 m s⁻¹. The friction dependence of these piston skirt and cylinder liner materials was studied as a function of load, sliding speed and temperature. Specifically, an increase in the sliding speed led to a decrease in the friction coefficient below approximately 70°C, while above this temperature, an increase in sliding speed led to an increase in the friction coefficient. The presence of a coating played an important role. It was found that the graphite/resin coating wore quickly, preventing the formation of a beneficial tribochemical film, while the a‐C : H coating exhibited a low friction coefficient and provided significant improvement over the uncoated samples. The effect of additives in the oils was also studied. The tribological behaviour of the interface was explained based on viscosity effects and subsequent changes in the lubrication regime, formation of chemical and tribochemical films. Copyright © 2010 John Wiley & Sons, Ltd.     

Output characteristics of a series three-port axial piston pump

Driving a hydraulic cylinder directly by a closed-loop hydraulic pump is currently a key research area in the field of electro-hydraulic control technology,and it is the most direct means to improve the energy efficiency of an electro-hydraulic control system.So far,this technology has been well applied to the pump-controlled symmetric hydraulic cylinder.However,for the differential cylinder that is widely used in hydraulic technology,satisfactory results have not yet been achieved,due to the asymmetric flow constraint.Therefore,based on the principle of the asymmetric valve controlled asymmetric cylinder in valve controlled cylinder technology,an innovative idea for an asymmetric pump controlled asymmetric cylinder is put forward to address this problem.The scheme proposes to transform the oil suction window of the existing axial piston pump into two series windows.When in use,one window is connected to the rod chamber of the hydraulic cylinder and the other is linked with a low-pressure oil tank.This allows the differential cylinders to be directly controlled by changing the displacement or rotation speed of the pumps.Compared with the loop principle of offsetting the area difference of the differential cylinder through hydraulic valve using existing technology,this method may simplify the circuits and increase the energy efficiency of the system.With the software SimulationX,a hydraulic pump simulation model is set up,which examines the movement characteristics of an individual piston and the compressibility of oil,as well as the flow distribution area as it changes with the rotation angle.The pump structure parameters,especially the size of the unloading groove of the valve plate,are determined through digital simulation.All of the components of the series arranged three distribution-window axial piston pump are designed,based on the simulation analysis of the flow pulse characteristics of the pump,and then the prototype pump is made.The basic characteristics,such as the pressure,flow and noise of the pumps under different rotation speeds,are measured on the test bench.The test results verify the correctness of the principle.The proposed research lays a theoretical foundation for the further development of a new pump-controlled cylinder system.